Apparatuses for reading from and/or writing to optical recording media which are suitable for different types of optical recording media often require a plurality of beam sources which emit light beams having a different wavelength. One example is apparatuses for reading from and/or writing to DVDs (digital versatile disc), which are usually able to reproduce CDs (compact disc). Whereas a wavelength of 650 nm is used for the DVDs, the CDs are read with a wavelength of 780 nm. The development of optical drives operating with even shorter wavelengths, such as e.g. drives for Blu-Ray Disc, which operate with a wavelength of 405 nm, will also necessitate the use of a plurality of beam sources in the future.
Many apparatuses use discrete laser diodes for generating the different wavelengths. However, many parts have to be used for such apparatuses, which is associated with high costs and a high adjustment outlay. High costs are caused especially by the required beam combination cube for combining the different light beams. An alternative is beam combiners based on optical gratings, as are described for example in WO 01/93254. However, gratings have a high degree of wavelength dependence, which can lead to problems if the wavelength of the laser diodes drifts with increasing temperature. Moreover, the gratings are not easy to produce since there are blazed gratings.
So-called twin laser diodes are increasingly being used instead of discrete laser diodes. In the case of twin laser diodes, two laser diodes, e.g. for DVD and CD are arranged in a common housing, which entails advantages for the parts costs and production. Twin photodetectors having discrete photodetectors for the two light beams are often used in this case. According to present-day prior art, this concept cannot be applied to all types of optical storage media since correspondingly rapid twin photodetectors still do not exist for example for the high speeds occurring in the case of DVD-ROM.